The present invention relates to a fixing device for use in an image forming apparatus such as an electrophotographic copier, a printer and a facsimile apparatus. Particularly, the present invention relates to a fixing device which prevents defects caused by excessive rise of temperature at the time of failure.
A conventional fixing device includes a fixing roller, a trembler coil for generating magnetic flux to heat the fixing roller, and a ferrite core around which the trembler coil is wound (see JP 08-286542A and JP 2001-23767A).
The ferrite core has a Curie temperature which is higher than a temperature at which the fixing roller performs normal fixing (hereinbelow referred to as fixing temperature).
The fixing operation of the conventional fixing device is described as follows. The fixing operation is performed by the fixing roller heated by the trembler coil. At that time, the ferrite core is heated by heat from the fixing roller.
Once the temperature of the ferrite core reaches the Curie temperature, the ferrite core becomes nonmagnetic, so that the magnetic flux from the trembler coil to the fixing roller is decreased and a heating value of the fixing roller is reduced. Thereby temperature rise of the fixing roller is suppressed.
The conventional fixing device has a temperature control mechanism for keeping the temperature of the fixing roller constant.
In the conventional fixing device, however, when the temperature control breaks down, a temperature uncontrolled state (i.e., power ON state) continues after an electric power is supplied, as shown in FIG. 9.
In this case, the temperature of the fixing roller is rapidly increased by electromagnetic induction from the trembler coil, whereas the temperature of the ferrite core is gradually increased by heat from the fixing roller.
The rate of temperature rise of the ferrite core is extremely lower than the rate of temperature rise of the fixing roller. Consequently, before the temperature of the ferrite core reaches the Curie temperature (250° C.), the temperature of the fixing roller reaches a temperature at which the fixing roller starts smoking (400° C.), and the fixing roller ignites before long.
In the case where a heat capacity of the fixing roller is reduced in order to reduce electric power consumption by shortening a warm-up completion time, the rate of temperature rise of the fixing roller becomes further higher than the rate of temperature rise of the ferrite core. This further increases the risk of the fixing roller to ignite.